On rearing in sauropods

["David Marjanovic" <david.marjanovic@gmx.at>]

Here's an abstract from the Darwin-Bernissart meeting (February 9 -- 13 this 
year, Brussels) that is relevant to the current discussion on sauropod neck 
mobility.

----------------------------

Heinrich Mallison: Rearing for food? Kinetic/dynamic modeling of 
bipedal/tripodal poses in sauropod dinosaurs, p. 63 in Pascal Godefroit & 
Olivier Lambert (eds): Tribute to Charles Darwin and Bernissart Iguanodons: 
New Perspectives on Vertebrate Evolution and Early Cretaceous Ecosystems

Popular art and movies often portray sauropods as able to rear into a 
bipedal or tripodal (with tail support) pose for feeding or inter- and 
intraspecific combat. Such suggestions can also be found in the scientific 
literature (e.g. Borsuk-Bialynicka, 1977; Bakker, 1986), and Alexander 
(1985) attempted to estimate rearing ability in diplodocids based on the 
position of their Center of Mass (CoM). Rearing would give sauropod[s] an 
enormous height range from which to select their food, although the long 
necks already give quadrupedal sauropods great reach. Although Stevens & 
Parrish (2005) argued that sauropod necks were fairly stiff, Christian & 
Dzemski (2007) concluded that their mobility was high. in either case, a 
bipedal pose would have greatly increased the maximum feeding height for 
non-brachiosaurid sauropods and open[ed] access to food sources beyond the 
reach of other animals.
       To date, no investigation of the biomechanics of sauropod rearing 
has been published. I used kinetic/dynamic computer modeling to assess the 
mechanics of attaining a bipedal pose, and the torques required to sustain 
them for a prolonged time in *Diplodocus* and *Brachiosaurus*. The two 
genera form the extremes of the wide range of sauropod _bauplans_, with 
short forelimbs in *Diplodocus* and the longest known forelimbs of all 
sauropods in *Brachiosaurus*. The retention of an upright pose was 
investigated using a quasi-static approach, and compared to extant elephants 
[which sometimes rear in the wild].
       Several osteological characters of titanosauriforms, some of which 
are also present in brachiosaurids, are supposedly connected to rearing 
ability: flared ilia, a wide range of motion of the femur in the acetabulum, 
and opisthocoelus [argh! He makes mistakes like a native speaker!] anterior 
caudals as well as a fused pubic symphysis (Borsuk-Bialynicka, 1977). Wilson 
& Carrano (1999) mentioned the wide gauge of titanosaurs as helpful for 
rearing. The modeling results confirm that a wide gauge indeed increases 
stability, but suggest that none of the others is connected to rearing. To 
the contrary, higher mobility in the post-cervical vertebral column may be 
detrimental, because it makes higher muscle forces in the axial musculature 
necessary. Aside from the extremely posterior position of the CoM, 
*Diplodocus* shows further adaptations that combine with upright feeding: 
the most mobile neck in Sauropoda, a proportionally large pelvic girdle for 
large muscles, and chevrons with a highly derived shape that allows 
supporting weight on them in the area of the tail that touches the ground 
when the animal is in an upright pose.
       Modeling of the rearing motion showed that *Brachiosaurus* would 
have had serious stability problems, due to the large moment arm of its CoM 
around the hip joint. In *Diplodocus* the CoM is located close to the hips, 
and slight imbalances do not result in large accelerations that threaten to 
topple the animal. The modeling results also indicate that diplodocids could 
sustain a bipedal/tripodal pose with significantly less effort than 
elephants, while *Brachiosaurus* would have to produce extreme torques in 
the limb joints. Sufficient muscle mass to produce these can not [sic] be 
placed on the bones, even assuming that dinosaurs were much more muscular 
than mammals. It therefore appears likely that diplodocids were the only 
sauropods capable of prolonged upright feeding.

----------------------------

When asked about the blood pressure problem, the author said that some kind 
of simple adaptation must have been present anyway just to let the animal 
lower the head to the ground and back to a horizontal neck posture.

Outside of his talk, he also talked a lot about the rest of his recently 
finished PhD thesis. We can look forward to a lot of very interesting and 
very, very, very, very well argued papers.